Aggregation of Resources in Enhanced Control Channels

ABSTRACT

Time-frequency resources in a single PRB pair are used for both frequency-localized transmissions as well as distributed transmissions. A first control message is transmitted ( 2710 ) to a first user equipment (UE) using a first PRB pair, using first subsets of resource elements that are aggregated in a frequency-localized manner and that are transmitted using a single antenna port. A second control message to a second UE is simultaneously transmitted ( 2710 ), also using the first PRB pair, using second subsets of resources that are aggregated in a frequency-distributed manner across the first PRB pair and one or more additional PRB pairs and that are transmitted using at least two antenna ports, including the single antenna port used to transmit the symbols in the first non-overlapping subsets of resource elements.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication Ser. No. 61/612,803, filed 19 Mar. 2012. The entire contentsof said U.S. Provisional application are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure is related to control channel signaling inwireless communication systems, and is more particularly related totechniques for aggregating transmission resources to form enhancedcontrol channel signals.

BACKGROUND

The 3^(rd)-Generation Partnership Project (3GPP) has developed athird-generation wireless communications known as Long Term Evolution(LTE) technology, as documented in the specifications for the EvolvedUniversal Terrestrial Radio Access Network (UTRAN). LTE is a mobilebroadband wireless communication technology in which transmissions frombase stations (referred to as eNodeBs or eNBs in 3GPP documentation) tomobile stations (referred to as user equipment, or UEs, in 3GPPdocumentation) are sent using orthogonal frequency division multiplexing(OFDM). OFDM splits the transmitted signal into multiple parallelsub-carriers in frequency.

More specifically, LTE uses OFDM in the downlink and Discrete FourierTransform (DFT)-spread OFDM in the uplink. The basic LTE downlinkphysical resource can be viewed as a time-frequency resource grid. FIG.1 illustrates a portion of the available spectrum of an exemplary OFDMtime-frequency resource grid 50 for LTE. Generally speaking, thetime-frequency resource grid 50 is divided into one millisecondsubframes. As seen in FIGS. 1 and 2, each subframe includes a number ofOFDM symbols. For a normal cyclic prefix (CP) length, which is suitablefor use in situations where multipath dispersion is not expected to beextremely severe, a subframe consists of fourteen OFDM symbols. Asubframe has only twelve OFDM symbols if an extended cyclic prefix isused. In the frequency domain, the physical resources are divided intoadjacent subcarriers with a spacing of 15 kHz. The number of subcarriersvaries according to the allocated system bandwidth. The smallest elementof the time-frequency resource grid 50 is a resource element. A resourceelement consists of one OFDM subcarrier during one OFDM symbol interval.

LTE resource elements are grouped into resource blocks (RBs), which inits most common configuration consists of 12 subcarriers and 7 OFDMsymbols (one slot). Thus, a RB typically consists of 84 REs. The two RBsoccupying the same set of 12 subcarriers in a given radio subframe (twoslots) are referred to as an RB pair, which includes 168 resourceelements if a normal CP is used. Thus, an LTE radio subframe is composedof multiple RB pairs in frequency with the number of RB pairsdetermining the bandwidth of the signal. In the time domain, LTEdownlink transmissions are organized into radio frames of 10milliseconds, each radio frame consisting of ten equally-sized subframesof length T_(subframe)=1 millisecond.

The signal transmitted by an eNB to one or more UEs may be transmittedfrom multiple antennas. Likewise, the signal may be received at a UEthat has multiple antennas. The radio channel between the eNB distortsthe signals transmitted from the multiple antenna ports. To successfullydemodulate downlink transmissions, the UE relies on reference symbolsthat are transmitted on the downlink. Several of these reference symbolsare illustrated in the resource grid 50 shown in FIG. 2. These referencesymbols and their position in the time-frequency resource grid are knownto the UE and hence can be used to determine channel estimates bymeasuring the effect of the radio channel on these symbols.

Several techniques may be used to take advantage of the availability ofmultiple transmit and/or receive antennas. Some of these are referred toas Multiple-Input Multiple-Output (MIMO) transmission techniques. Oneexample technique used when multiple transmit antennas are available iscalled “transmit precoding,” and involves the directional transmissionof signal energy towards a particular receiving UE. With this approach,the signal targeted to a particular UE is simultaneously transmittedover each of several antennas, but with individual amplitude and/orphase weights applied to the signal at each transmit antenna element.This application of weights to the signal is referred to as “precoding,”and the antenna weights for a particular transmission can mathematicallybe described in a comprehensive way by a precoding vector.

This technique is sometimes referred to as UE-specific precoding. Thereference symbols accompanying a precoded transmission and used for itsdemodulation are denoted a UE-specific reference signal (UE-specificRS). If the transmitted symbols making up a UE-specific RS in a given RBare precoded with the same UE-specific precoding as the data carried inthat RB (where data in this sense can be control information), then thetransmission of the UE-specific RS and data can treated as though theywere performed using a single virtual antenna, i.e. a single antennaport. The targeted UE performs channel estimation using the UE-specificRS and uses the resulting channel estimate as a reference fordemodulating the data in the RB.

UE-specific RS are transmitted only when data is transmitted to a UE inthe RB pair, and are otherwise not present. In Releases 8, 9, and 10 ofthe LTE specifications, UE-specific reference signals are included aspart of each of the RBs that are allocated to a UE for demodulation ofthe physical downlink shared data channel (PDSCH). Release 10 of the LTEspecifications also supports spatial multiplexing of the downlinktransmission, allowing up eight spatially multiplexed “layers” to betransmitted simultaneously. Accordingly, there are eight orthogonalUE-specific RS, which are described in the 3GPP document “EvolvedUniversal Terrestrial Radio Access (E-UTRA); Physical Channels andModulation,” 3GPP TS 36.211, v. 10.0.0 (December 2012), available atwww.3gpp.org. These are denoted as antenna ports 7-15. FIG. 3 shows anexample of the mapping of UE-specific reference symbols to a RB pair; inthis example antenna ports 7 and 9 are shown. Antenna ports 8 and 10 canbe obtained as code-division multiplexed reference signals on top ofantenna ports 7 and 9, respectively.

Another type of reference symbols are those that can be used by all UEs.These reference symbols must therefore have wide cell area coverage andare thus not precoded towards any particular UE. One example is thecommon reference symbols (CRS) used by UEs for various purposes,including channel estimation and mobility measurements. These CRS aredefined so that they occupy certain pre-defined REs within all thesubframes in the system bandwidth, irrespectively of whether there isany data being sent to users in a subframe or not. These CRS are shownas “reference symbols” in FIG. 2.

Another type of reference symbol is the channel state information RS(CSI-RS), introduced in Release 10 of the LTE specifications. CSI-RS areused for measurements associated with precoding matrix and transmissionrank selection for transmission modes that use the UE-specific RSdiscussed above. The CSI-RS are also UE-specifically configured. Yetanother type of RS is the positioning RS (PRS), which was introduced inLTE Release 9 to improve positioning of UEs in a network.

Messages transmitted over the radio link to users can be broadlyclassified as control messages or data messages. Control messages areused to facilitate the proper operation of the system as well as properoperation of each UE within the system. Control messages includecommands to control functions such as the transmitted power from a UE,signaling of RBs within which the data is to be received by the UE ortransmitted from the UE, and so on.

Specific allocations of time-frequency resources in the LTE signal tosystem functions are referred to as physical channels. For example, thephysical downlink control channel (PDCCH) is a physical channel used tocarry scheduling information and power control messages. The physicalHARQ indicator channel (PHICH) carries ACK/NACK in response to aprevious uplink transmission, and the physical broadcast channel (PBCH)carries system information. The primary and secondary synchronizationsignals (PSS/SSS) can also be seen as control signals, and have fixedlocations and periodicity in time and frequency so that UEs thatinitially access the network can find them and synchronize. Similarly,the PBCH has a fixed location relative to the primary and secondarysynchronization signals (PSS/SSS). The UE can thus receive the systeminformation transmitted in BCH and use that system information to locateand demodulate/decode the PDCCH, which carries control informationspecific to the UE.

As of Release 10 of the LTE specifications, all control messages to UEsare demodulated using channel estimates derived from the commonreference signals (CRS). This allows the control messages to have acell-wide coverage, to reach all UEs in the cell without the eNB havingany particular knowledge about the UEs' positions. Exceptions to thisgeneral approach are the PSS and SSS, which are stand-alone signals anddo not require reception of CRS before demodulation. The first one tofour OFDM symbols of the subframe are reserved to carry such controlinformation; one OFDM symbol is used for this purpose in the examplesubframe shown in FIG. 2, where the control region may contain up tothree OFDM symbols for control signaling. The actual number of OFDMsymbols reserved to the control region may vary, depending on theconfiguration of a particular cell.

Control messages can be categorized into messages that need to be sentonly to one UE (UE-specific control) and those that need to be sent toall UEs or some subset of UEs numbering more than one (common control)within the cell being covered by the eNB. Messages of the first type(UE-specific control messages) are typically sent using the PDCCH, usingthe control region. It should be noted that in future LTE releases therewill be new carrier types that may not have such a control region, i.e.,that do not have PDCCH transmissions. These new carrier types may noteven include CRS, and are therefore not backward compatible. A newcarrier of this type is introduced in Release 11. However, this newcarrier type is used only in a carrier aggregation scenario, and isalways aggregated with a legacy (backward-compatible) carrier type. Infuture releases of LTE it may also be possible to have stand-alonecarriers that do not have a control region and that are not associatedwith a legacy carrier.

Control messages of PDCCH type are transmitted in association with CRS,which are used by the receiving mobile terminals to demodulate thecontrol message. Each PDCCH is transmitting using resource elementsgrouped into units called control channel elements (CCEs) where each CCEcontains 36 REs. A single PDCCH message may use more than one CCE; inparticular a given PDCCH message may have an aggregation level (AL) of1, 2, 4 or 8 CCEs. This allows for link adaptation of the controlmessage. Each CCE is mapped to 9 resource element groups (REGs)consisting of 4 RE each. The REGs for a given CCE are distributed overthe system bandwidth to provide frequency diversity for a CCE. This isillustrated in FIG. 4. Hence, a PDCCH message can consist of up to 8CCEs spanning the entire system bandwidth in the first one to four OFDMsymbols, depending on the configuration.

Processing of a PDCCH message in an eNB begins with channel coding,scrambling, modulation, and interleaving of the control information. Themodulated symbols are then mapped to the resource elements in thecontrol region. As mentioned above, control channel elements (CCE) havebeen defined, where each CCE maps to 36 resource elements. By choosingthe aggregation level, link-adaptation of the PDCCH is obtained. Intotal there are N_(CCE) CCEs available for all the PDCCH to betransmitted in the subframe; the number N_(CCE) may vary from subframeto subframe, depending on the number of control symbols n and the numberof configured PHICH resources.

Since N_(CCE) can vary from subframe to subframe, the receiving terminalmust blindly determine the position of the CCEs for a particular PDCCHas well as the number of CCEs used for the PDCCH. With no constraints,this could be a computationally intensive decoding task. Therefore, somerestrictions on the number of possible blind decodings a terminal needsto attempt have been introduced, as of Release 8 of the LTEspecifications. One constraint is that the CCEs are numbered and CCEaggregation levels of size K can only start on CCE numbers evenlydivisible by K. This is shown in FIG. 5, which illustrates CCEaggregation for aggregation levels AL-1, AL-2, AL-4, and AL-8. Forexample, an AL-8 PDCCH message, made up of eight CCEs, can only begin onCCEs numbered 0, 8, 16, and so on.

A terminal must blindly decode and search for a valid PDCCH over a setof CCEs referred to as the UE's search space. This is the set of CCEsthat a terminal should monitor for scheduling assignments or othercontrol information, for a given AL. An example search space isillustrated in FIG. 6, which illustrates the search space a particularterminal needs to monitor. Note that different CCEs must be monitoredfor each AL. In total there are NCCE=15 CCEs in this example. A commonsearch space, which must be monitored by all mobile terminals, is markedwith diagonal stripes, while a UE-specific search space is shaded.

In each subframe and for each AL, a terminal will attempt to decode allof the candidate PDCCHs that can be formed from the CCEs in its searchspace. If the Cyclic Redundancy Check (CRC) for the attempted decodingchecks out, then the contents of the candidate PDCCH are assumed to bevalid for the terminal, and the terminal further processes the receivedinformation. Note that two or more terminals may have overlapping searchspaces, in which case the network may have to select only one of themfor scheduling of the control channel. When this happens, thenon-scheduled terminal is said to be blocked. The search spaces for a UEvary pseudo-randomly from subframe to subframe to reduce this blockingprobability.

As suggested by FIG. 6, the search space is further divided into acommon part and a terminal-specific (or UE-specific) part. In the commonsearch space, PDCCH containing information to all or a group ofterminals is transmitted (paging, system information, etc.). If carrieraggregation is used, a terminal will find the common search spacepresent on the primary component carrier (PCC) only. The common searchspace is restricted to aggregation levels 4 and 8, to give sufficientchannel code protection for all terminals in the cell. Note that sinceit is a broadcast channel, link adaptation cannot be used. The m8 and m4first PDCCH (where the “first” PDCCH is the one having the lowest CCEnumber) in an AL of 8 or 4, respectively, belong to the common searchspace. For efficient use of the CCEs in the system, the remaining searchspace is terminal specific at each aggregation level.

A CCE consists of 36 QPSK modulated symbols that map to 36 REs that areunique to the given CCE. Hence, knowing the CCE means that the REs arealso known automatically. To maximize the diversity and interferencerandomization, interleaving is used before a cell-specific cyclic shiftand mapping to REs. Note that in most cases some CCEs are empty, due toPDCCH location restrictions within terminal search spaces andaggregation levels. The empty CCEs are included in the interleavingprocess and mapped to REs as any other PDCCH, to maintain the searchspace structure. Empty CCEs are set to zero power, meaning that thepower that would have otherwise been used may be allocated instead tonon-empty CCEs, to further enhance the PDCCH transmission.

To facilitate the use of 4-antenna transmit diversity, each group offour adjacent QPSK symbols in a CCE is mapped to four adjacent REs,denoted a RE group (REG). Hence, the CCE interleaving is quadruplex-(group of 4) based. The mapping process has a granularity of 1 REG, andone CCE corresponds to nine REGs (36 RE).

Transmission of the physical downlink shared data channel (PDSCH) to UEsuses those REs in a RB pair that are not used either for controlmessages (i.e, in the data region of FIG. 4) or RS. The PDSCH can betransmitted using either UE-specific RS or the CRS as a demodulationreference, depending on the PDSCH transmission mode. The use ofUE-specific RS allows a multi-antenna base station to optimize thetransmission using pre-coding of both data and reference signals beingtransmitted from the multiple antennas so that the received signalenergy increases at the UE and consequently, the channel estimationperformance is improved and the data rate of the transmission could beincreased.

For Release 11 of the LTE specifications, it has been agreed tointroduce UE-specific transmission of control information in the form ofenhanced control channels. This is done by allowing the transmission ofcontrol messages to a UE where the transmissions are placed in the dataregion of the LTE subframe and are based on UE-specific referencesignals. Depending on the type of control message, the enhanced controlchannels formed in this manner are referred to as the enhanced PDCCH(ePDCCH), enhanced PHICH (ePHICH), and so on.

For the enhanced control channel in Release 11, it has been furtheragreed to use antenna port pε{107,108,109,110} for demodulation, whichcorrespond with respect to reference symbol positions and set ofsequences to antenna ports pε{7,8,9,10}, i.e., the same antenna portsthat are used for data transmissions on the Physical Data Shared Channel(PDSCH), using UE-specific RS. This enhancement means that the precodinggains already available for data transmissions can be achieved for thecontrol channels as well. Another benefit is that different physical RBpairs (PRB pairs) for enhanced control channels can be allocated todifferent cells or to different transmission points within a cell. Thiscan be seen in FIG. 7, which illustrates ten RB pairs, three of whichare allocated to three separate ePDCCH regions comprising one PRB paireach. Note that the remaining RB pairs can be used for PDSCHtransmissions. The ability to allocate different PRB pairs to differentcells or different transmission points facilitates inter-cell orinter-point interference coordination for control channels. This isespecially useful for heterogeneous network scenarios, as will bediscussed below.

The same enhanced control region can be used simultaneously by differenttransmission points within a cell or by transmission points belonging todifferent cells, when those points are not highly interfering with oneanother. A typical case is the shared cell scenario, an example of whichis illustrated in FIG. 8. In this case, a macro cell 62 contains severallower power pico nodes A, B, and C within its coverage area 68, the piconodes A, B, C having (or being associated with) the same synchronizationsignal/cell ID. In pico nodes which are geographically separated, as isthe case with pico nodes B and C in FIG. 8, the same enhanced controlregion, i.e., the same PRBs used for the ePDCCH, can be re-used. Withthis approach, the total control channel capacity in the shared cellwill increase, since a given PRB resource is re-used, potentiallymultiple times, in different parts of the cell. This ensures that areasplitting gains are obtained. An example is shown in FIG. 9, which showsthat pico nodes B and C share the enhanced control region whereas A, dueto its proximity to both B and C, is at risk of interfering with theother pico nodes and is therefore assigned an enhanced control regionwhich is non-overlapping. Interference coordination between pico nodes Aand B, or equivalently transmission points A and B, within a shared cellis thereby achieved. Note that in some cases, a UE may need to receivepart of the control channel signaling from the macro cell and the otherpart of the control signaling from the nearby Pico cell.

This area splitting and control channel frequency coordination is notpossible with the PDCCH, since the PDCCH spans the whole bandwidth.Further, the PDCCH does not provide possibility to use UE-specificprecoding since it relies on the use of CRS for demodulation.

FIG. 10 shows an ePDCCH that is divided into multiple groups and mappedto one of the enhanced control regions. This represents a “localized”transmission of the ePDCCH, since all of the groups making up the ePDCCHmessage are grouped together in frequency. Note that these multiplegroups are similar to the CCEs in the PDCCH, but are not necessarilymade up of the same numbers of REs. Also note that, as seen in FIG. 10,the enhanced control region does not start at OFDM symbol zero. This isto accommodate the simultaneous transmission of a PDCCH in the subframe.However, as was mentioned above, there may be carrier types in futureLTE releases that do not have a PDCCH at all, in which case the enhancedcontrol region could start from OFDM symbol zero within the subframe.

While the localized transmission of ePDCCH illustrated in FIG. 10enables UE-specific precoding, which is an advantage over theconventional PDCCH, in some cases it may be useful to be able totransmit an enhanced control channel in a broadcasted, wide areacoverage fashion. This is particularly useful if the eNB does not havereliable information to perform precoding towards a certain UE, in whichcase a wide area coverage transmission may be more robust. Another casewhere distributed transmission may be useful is when the particularcontrol message is intended for more than one UE, since in this caseUE-specific precoding cannot be used. This is the general approach takenfor transmission of the common control information using PDCCH (i.e. inthe common search space (CSS)).

Accordingly, a distributed transmission over enhanced control regionscan be used, instead of the localized transmission shown in FIG. 10. Anexample of distributed transmission of the ePDCCH is shown in FIG. 11,where the four parts belonging to the same ePDCCH are distributed amongthe enhanced control regions.

3GPP has agreed that both localized and distributed transmission of anePDCCH should be supported, these two approaches corresponding generallyto FIGS. 10 and 11, respectively. When distributed transmission is used,then it is also beneficial if antenna diversity can be achieved tomaximize the diversity order of an ePDCCH message. On the other hand,sometimes only wideband channel quality and wideband precodinginformation are available at the eNB, in which case it could be usefulto perform a distributed transmission but with UE-specific, wideband,precoding.

Several problems relate to the use of the ePDCCH. For example, if anePDCCH based on distributed transmission is mapped to all PRB pairs thathave been configured for the UE, then it is currently a problem thatunused resources in these PRB pairs cannot be simultaneously used forPDSCH transmission. As a result, a large control channel overhead willoccur in the event that the fraction of unused resources is large.Another unsolved problem is how to handle the collisions betweenenhanced control channels and the legacy reference signals such asCSI-RS, CRS, PRS, PSS, SSS and legacy control channels such as PDCCH,PHICH, PCFICH and PBCH.

More generally, remaining challenges include how to design the searchspace for ePDCCH reception in an efficient manner, so that both thelocalized and distributed (or UE-specific precoding and diversitytransmission) ePDCCH can be supported flexibly for different ePDCCHtransmissions.

SUMMARY

According to several embodiments detailed herein, the time-frequencyresources in a single PRB pair can be flexibly used by the base stationfor both frequency-localized transmissions as well as distributedtransmissions. These embodiments include methods for transmittingdownlink control information in a radio communication system, wherebylocalized and distributed transmissions of control channel messages mayutilize the same PRB pair or pairs. An example method includestransmitting a first control message to a first user equipment (UE)using a first PRB pair, where the first control message is divided amongtwo or more first non-overlapping subsets of resource elements, at leastone of which is in the first PRB pair. These two or more firstnon-overlapping subsets of resource elements are aggregated in afrequency-localized manner within at least the first PRB pair, andsymbols in these two or more first non-overlapping subsets of resourceelements are transmitted using a single antenna port. The method furtherincludes simultaneously transmitting a second control message to asecond UE, also using the first PRB pair, where the second controlmessage is divided among two or more second non-overlapping subsets ofresource elements, at least one of which is in the first PRB pair. Inthis case, the two or more second non-overlapping subsets of resourceelements are aggregated in a frequency-distributed manner across thefirst PRB pair and one or more additional PRB pairs, and symbols in atleast two of the two or more second non-overlapping subsets of resourceelements are transmitted using differing antenna ports. These differingantenna ports include the single antenna port used to transmit thesymbols in the two or more first non-overlapping subsets of resourceelements. In some embodiments of this example method, each of thenon-overlapping subsets of resources defines an enhanced REG (eREG) inan LTE network.

Other embodiments include base station and user equipment adapted tocarry out the methods summarized above, and variants thereof. Of course,the techniques, systems, and apparatus described herein are not limitedto the above features and advantages. Indeed, those skilled in the artwill recognize additional features and advantages upon reading thefollowing detailed description, and upon viewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the time-frequency resource grid of an OFDM signal.

FIG. 2 illustrates a subframe of an LTE signal with one OFDM symbol ofcontrol signaling.

FIG. 3 illustrates an example mapping of UE-specific reference symbolsto a PRB pair.

FIG. 4 illustrates the mapping of a CCE to the control region of an LTEsubframe.

FIG. 5 illustrates the aggregation of CCEs into control channelmessages.

FIG. 6 illustrates an example search space.

FIG. 7 illustrates the mapping of an example enhanced control channelregion to an LTE subframe.

FIG. 8 illustrates an example heterogeneous network.

FIG. 9 illustrates an allocation of ePDCCH to pico nodes in aheterogeneous network.

FIG. 10 illustrates the localized mapping of an ePDCCH to an enhancedcontrol region.

FIG. 11 illustrates the distributed mapping of an ePDCCH to enhancedcontrol regions.

FIG. 12 illustrates an example radio communications network in whichseveral of the presently disclosed techniques may be applied.

FIG. 13 illustrates an example mapping of eREGs and antenna ports to aPRB pair.

FIG. 14 shows one possible association of antenna ports to eREGs for theexample mapping of FIG. 13.

FIG. 15 shows another possible association of antenna ports to eREGs forthe example mapping of FIG. 13.

FIG. 16 illustrates an example of antenna port associations fordiversity transmission for an example PRB pair that includes eight eREGsand four antenna ports.

FIG. 17 illustrates an example of antenna port associations forUE-specific precoding transmission for an example PRB pair that includeseight eREGs and four antenna ports.

FIG. 18 illustrates an example of antenna port associations for bothdiversity transmission and UE-specific precoding transmission in asingle PRB pair.

FIG. 19 illustrates a two-dimensional representation of a search spacethat includes both localized and distributed aggregations of CCEs.

FIG. 20 shows elements of an example wireless network where a UE mayreceive ePDCCH transmissions from one or both of two nodes.

FIG. 21 is another representation of a search space, where the PRB pairsin the search space are divided into two groups.

FIGS. 22-26 illustrate other examples of two-group search spaces.

FIG. 27 is a process flow diagram illustrating another example methodfor transmitting control information.

FIG. 28 is process flow diagram illustrating an example method forreceiving downlink control information.

FIG. 29 is a block diagram of an example base station adapted to carryout one or more of the techniques described herein.

FIG. 30 is a block diagram of an example user equipment (UE) adapted tocarry out one or more of the techniques described herein.

DETAILED DESCRIPTION

In the discussion that follows, specific details of particularembodiments of the presently disclosed techniques and apparatus are setforth for purposes of explanation and not limitation. It will beappreciated by those skilled in the art that other embodiments may beemployed apart from these specific details. Furthermore, in someinstances detailed descriptions of well-known methods, nodes,interfaces, circuits, and devices are omitted so as not to obscure thedescription with unnecessary detail. Those skilled in the art willappreciate that the functions described may be implemented in one or inseveral nodes. Some or all of the functions described may be implementedusing hardware circuitry, such as analog and/or discrete logic gatesinterconnected to perform a specialized function, ASICs, PLAs, etc.Likewise, some or all of the functions may be implemented using softwareprograms and data in conjunction with one or more digitalmicroprocessors or general purpose computers. Where nodes thatcommunicate using the air interface are described, it will beappreciated that those nodes also have suitable radio communicationscircuitry. Moreover, the technology can additionally be considered to beembodied entirely within any form of computer-readable memory, includingnon-transitory embodiments such as solid-state memory, magnetic disk, oroptical disk containing an appropriate set of computer instructions thatwould cause a processor to carry out the techniques described herein.

Hardware implementations may include or encompass, without limitation,digital signal processor (DSP) hardware, a reduced instruction setprocessor, hardware (e.g., digital or analog) circuitry including butnot limited to application specific integrated circuit(s) (ASIC) and/orfield programmable gate array(s) (FPGA(s)), and (where appropriate)state machines capable of performing such functions.

In terms of computer implementation, a computer is generally understoodto comprise one or more processors or one or more controllers, and theterms computer, processor, and controller may be employedinterchangeably. When provided by a computer, processor, or controller,the functions may be provided by a single dedicated computer orprocessor or controller, by a single shared computer or processor orcontroller, or by a plurality of individual computers or processors orcontrollers, some of which may be shared or distributed. Moreover, theterm “processor” or “controller” also refers to other hardware capableof performing such functions and/or executing software, such as theexample hardware recited above.

References throughout the specification to “one embodiment” or “anembodiment” mean that a particular feature, structure, or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the present invention. Thus, the appearance of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthe specification are not necessarily all referring to the sameembodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Referring once again to the drawings, FIG. 12 illustrates an exemplarymobile communication network 10 for providing wireless communicationservices to mobile stations 100. Three mobile stations 100, which arereferred to as “user equipment” or “UE” in LTE terminology, are shown inFIG. 10. The mobile stations 100 may comprise, for example, cellulartelephones, personal digital assistants, smart phones, laptop computers,handheld computers, or other devices with wireless communicationcapabilities. It should be noted that the terms “mobile station” or“mobile terminal,” as used herein, refer to a terminal operating in amobile communication network and do not necessarily imply that theterminal itself is mobile or moveable. Thus, the terms may refer toterminals that are installed in fixed configurations, such as in certainmachine-to-machine applications, as well as to portable devices, devicesinstalled in motor vehicles, etc.

The mobile communication network 10 comprises a plurality of geographiccell areas or sectors 12. Each geographic cell area or sector 12 isserved by a base station 20, which is generally referred to in LTE as anEvolved NodeB (eNodeB). One base station 20 may provide service inmultiple geographic cell areas or sectors 12. The mobile stations 100receive signals from base station 20 on one or more downlink (DL)channels, and transmit signals to the base station 20 on one or moreuplink (UL) channels.

For illustrative purposes, several embodiments will be described in thecontext of context of E-UTRAN, also referred to as LTE. It should beunderstood that the problems and solutions described herein are equallyapplicable to wireless access networks and user equipment (UEs)implementing other access technologies and standards. LTE is used as anexample technology where the invention is suitable, and using LTE in thedescription therefore is particularly useful for understanding theproblem and solutions solving the problem. Those skilled in the art willappreciate, however, that the presently disclosed techniques may be moregenerally applicable to other wireless communication systems, including,for example, WiMax (IEEE 802.16) systems. The use of LTE terminology todescribe the various embodiments detailed below should thus not be seenas limiting to this particular technology.

As noted above, 3GPP has agreed that both distributed and localizedtransmission of an ePDCCH should be supported in forthcoming releases ofthe standards for LTE, these two approaches corresponding generally toFIGS. 10 and 11, respectively. When distributed transmission is used,then it is also generally beneficial if antenna diversity can beachieved to maximize the diversity order of an ePDCCH message. On theother hand, sometimes only wideband channel quality and widebandprecoding information are available at the eNB, in which case it couldbe useful to perform a distributed transmission but with UE-specific,wideband, precoding.

As of release 11 of the LTE specifications, the enhanced control channelwill use UE-specific RS (e.g., as shown in FIG. 3) as the reference fordemodulation. A given ePDCCH will use one, some or all of antenna ports7, 8, 9, and 10 for demodulation, depending on the number of antennaports needed in a RB pair.

Each PRB pair used for the enhanced control channel can be divided intovarious groups of time-frequency resources, denoted enhanced or extendedresource element groups (eREGs), or enhanced CCEs (eCCEs). In localizedePDCCH transmission, each group of time-frequency resources isassociated with a unique RS from the set of UE-specific RS, orequivalently antenna port, which is located in the same RB or RB pair.For instance, when a UE demodulates the information in a given eREG ofthe RB or RB pair, it uses the RS/antenna port associated with thateREG. Furthermore, each resource in an RB or RB pair can beindependently assigned to UEs.

FIG. 13 shows an example of one possible grouping, illustrating adownlink RB pair with four enhanced resource element groups (eREG), eacheREG consisting of 36 RE. Each eREG is associated with an antenna port(AP). In this example, each AP is associated with two eREG. Antennaports using the same resource elements (e.g., ports 7 and 8) are madeorthogonal by the use of orthogonal cover codes (OCC).

FIG. 14 illustrates an association of AP to eREG for the example of FIG.13. Here it can be seen that eREG 1 and eREG 3 are each associated withAP 7, while eREG2 and eREG4 are associated with AP 9. When a UEdemodulates part of an ePDCCH transmitted in eREG1, for example, it willuse the RS associated with AP 7 for demodulation. When a UE demodulatesan ePDCCH transmitted in eREG1 and eREG2, it will use both AP7 and AP9for demodulation of the corresponding part of the ePDCCH message. Inthis way, antenna diversity can be obtained for the ePDCCH if multipleantennas are available at the eNB and if AP7 and AP9 are mapped todifferent antennas.

Note that even if multiple orthogonal RS are used in the RB or RB pair,there is only a single layer of control data transmitted. As can be seenin FIG. 14, it is possible that more than one eREG are using a given AP,which is possible since the eREG are orthogonal in the time-frequencyOFDM grid. Referring again to FIGS. 13 and 14, for example, eREG1 andeREG3 are associated with the same antenna port and thus are transmittedwith the same precoding vector. Note that if a given ePDCCH uses all theeREGs in a PRB pair configured according to FIGS. 13 and 14, thenantenna diversity or precoding beam diversity can be achieved. This canbe useful in the event the preferred precoding vector is unknown at thebase station side, or if the control message carried by the ePDCCH isintended for multiple UEs (e.g. a common control channel).

Alternatively, if the ePDCCH uses all the eREGs in a PRB pair and thebase station chooses to perform precoding to a single UE, i.e., with thesame precoder applied to all eREGs in the PRB pair, then only oneantenna port needs to be used and UE-specific precoding can then beapplied to the whole ePDCCH message. The related node diagram for thisscenario is shown in FIG. 15, which illustrates an association of AP toeREG in the event that all eREGs are being used for the same UE and thusonly AP7 needs to be used (AP9 is unused).

Control information can be transmitted in each group of time-frequencyresources, or eREG. The control information in a given eREG may consistof, but is not limited to, all or part of an enhanced PDCCH, all or partof a CCE, or all or part of an enhanced PHICH or enhanced PBCH. If theresource/eREG is too small to contain a whole enhanced PDCCH, CCE, PHICHor PBCH, a fraction can be transmitted in the eREG and the remainingportion in other eREGs, in the same RB or RB pair or in RB pairselsewhere in the same subframe.

If an ePDCCH for distributed transmission is mapped to all PRB pairsthat have been configured for the UE, then it is a problem, according tocurrent 3GPP agreements, that unused resources in these PRB pairs cannotbe simultaneously used for PDSCH transmission. As a result, a largecontrol channel overhead will occur in the event that the fraction ofunused resources is large. Another problem is how to handle thecollisions between enhanced control channels and the legacy referencesignals such as CSI-RS, CRS, PRS, PSS, SSS and legacy control channelssuch as PDCCH, PHICH, PCFICH and PBCH.

As noted above, a more general problem is how to design the search spacefor ePDCCH reception in an efficient manner, so that both localized anddistributed (or UE-specific precoding and diversity transmission) ePDCCHcan be supported flexibly for different ePDCCH transmissions. Hence, itis a problem how a CCE or other group of time-frequency resources can beused for either localized transmission or distributed transmissionwithout need for RRC reconfiguration. In other words, it is a problemhow to have flexible use of a CCE or other group of time-frequencyresources without configuring each one semi-statically to be of one orthe other type.

It is further a problem how to define the search space so that the UEcan receive the ePDCCH from more than one transmission point in atransparent manner, and possibly with only low capacity backhaul betweenthe transmission points, which implies that only semi-static ePDCCHcoordination between these transmission points is possible.

According to various embodiments, examples of which are described below,the PRB pairs configured for use in transmitting and receiving an ePDCCH(i.e., the PRB pairs making up a UE's search space) are divided into oneor more groups of PRB pairs, where:

-   -   A given ePDCCH is mapped to resource elements within one such        group only;    -   The association between CCE, eREG, or other group of        time-frequency resources is arranged so that a particular group        of time-frequency resources can be used flexibly for either        UE-specific precoding or diversity transmission, depending on        configuration and/or depending whether the search space where        the ePDCCH is received is common or UE-specific; and    -   Localized and distributed ePDCCH transmission can be received        within a group depending on the how the aggregation of resources        within the group is performed.

In some embodiments, a common search space may be mapped to only one ofthe groups of PRB pairs; this group could be denoted the primary group.The primary group's location can be signaled to the UE in one of theinformation blocks, such as the MIB transmitted in the PBCH. Thisprimary group could be used for initial access to the system and foraccess to CSS on a stand-alone carrier that does not have legacy PDCCHtransmitted

In some embodiments, the size of a group in terms of number of PRB pairsdepends on the level of puncturing and may differ from subframe tosubframe. In still other embodiments, groups of resources within a givenPRB pair or pairs may be grouped, i.e., aggregated, in a manner thatdepends of the level of puncturing for the PRB pairs.

These and other features of various embodiments will now be furtherdescribed.

To provide ePDCCH transmission to a given UE, a number K_ePDCCH of PRBpairs out of a total of K′ PRB pairs available to the UE are configuredto the UE. (K′ may be the number of PRB pairs in the system bandwidthfor that particular carrier or a UE-specific bandwidth in terms ofnumber of PRB pairs within the carrier.) In other words, the UE isinformed that a particular set of K_ePDCCH PRB pairs are allocated forenhanced control channel use.

As noted above, the resource elements in the PRB pairs may be groupedinto one or more sets of non-overlapping groups of resource elements.Thus, for example, the total number of available control channelelements (CCE) in the enhanced control region for a UE may beN_CCE-ePDCCH; these CCEs are mapped to resource elements in the K_ePDCCHPRB pairs in a specified manner that is known at both the eNB and theUE.

The available resource elements (REs) in each PRB pair may be furtherdivided into non-overlapping subsets of REs, which may be denotedextended resource element groups (eREG) or enhanced CCE (eCCE), or thelike. In the following description in this specification, the term“eREG” will be used to refer to such a subset, but it will beappreciated that “eCCE” or any other name can be used instead fordenoting such a subset of REs. In one example, a PRB pair is dividedinto eight eREG, where each eREG contains 18 RE, a total of 144 RE. APRB pair in a subframe with normal cyclic prefix contains 168 RE and theremaining 24 REs in the PRB pair contains the demodulation referencesignals (DMRS), in form of antenna port 7, 8, 9 and 10. In anotherexample, each eREG contains 9 REs. In this case, a PRB pair contains 16eREGs, in addition to the 24 REs set aside for the DMRS. Other sizes andarrangements of eREGs are possible.

As noted above in the background section, when UE-specific precoding ofan ePDCCH is used, each eREG is associated with an antenna port and thecorresponding reference symbols in the same PRB pair as the eREG. When aUE demodulates an ePDCCH that uses that eREG, the reference symbols forthe associated antenna port are used for channel estimation. Hence, theantenna port association is implicit. When an ePDCCH maps to multipleeREG within the same PRB pair, then multiple antenna ports may beassociated with these eREGs. Depending on whether UE-specific precodingor diversity transmission is configured for this ePDCCH, only a subsetof these associated antenna ports may be used to demodulate the ePDCCH.As an example, when UE-specific precoding is used, then only one ofthese associated AP may be used by the UE and if diversity transmissionis used, then two may be used.

An example of antenna port association for a configuration where eachPRB pair includes 8 eREGS is shown in FIG. 16. In this example, the 8eREGs of the PRB pair map to four antenna ports and are grouped intoseveral ePDCCHs. In this example, a first ePDCCH includes eREG 1 and 2,where eREG 1 is mapped to AP7 and eREG 2 is mapped to APB. (Note thatthis same ePDCCH may include eREGs in additional PRB pairs as well).Antenna port diversity is obtained for this ePDCCH since the two usedeREGs are transmitted via two different antenna ports. In the same PRB,another ePDCCH uses eREGs 5-8 and achieves two-fold diversity by the useof antenna port 9 and 10.

Given the same PRB pair, UE-specific precoding could alternatively beused, in which case only a single AP is associated with each group ofeREGs that belong to the same ePDCCH within the PRB pair. An example ofthis is shown in FIG. 17, where antenna port associations forUE-specific precoding transmission are shown in an example PRB thatagain includes eight eREGs and four antenna ports. One ePDCCH includeseREGs 1 and 2, which are both mapped to AP7. Another ePDCCH includeseREGs 3 and 4, which are mapped to AP8. Still another ePDCCH includeseREGS 5, 6, 7, and 8, which are all mapped to AP9.

It may be noted that in the examples given in FIGS. 16 and 17, each eREGin the lower set of four eREGs in each case is associated to either AP9or AP10, while each eREG in the upper set is associated to either AP7 orAP8. Thus, two independent and frequency-division-multiplexed set ofantenna ports are separately associated with the upper and lower sets ofeREGs. This makes it possible to have different transmission modes(UE-specific precoding or diversity) apply to groupings of eREGs in theupper and lower sets. Thus, for example, it is possible to map eREGs 1-4to AP7 and AP8 in the manner shown in FIG. 16, to achieve antennadiversity for the corresponding ePDCCHs, while simultaneously using themapping shown in FIG. 17 for eREGs 5-8, to utilize UE-specific precodingfor the corresponding ePDCCH.

One problem with this arrangement, however, is that if the CCE that mapsto eREGs 1 and 2 is using diversity then eREGs 3 and 4 also usediversity. The same goes for UE-specific precoding. Accordingly, itwould be beneficial to have even more flexibility between UE-specificand diversity transmission within the same PRB pair. A solution is givenin the embodiment described below.

Given an association between eREGs and APs like that shown in FIG. 16and FIG. 17 above, a first embodiment provides the flexibility to haveboth UE-specific precoding and diversity transmission in the same PRBpair, by enabling them to co-exist within the same set of eREGs (upperand lower sets respectively). An example of this is shown in FIG. 18,which illustrates antenna port associations for both UE-specificprecoding transmission and diversity transmission in the same PRB pair.More specifically, in the example of FIG. 18, the ePDCCH using eREGs 1and 2 is using UE-specific precoding and thus only one antenna port,while the ePDCCH that uses eREGs 3 and 4 is using diversitytransmission. Since eREGs 1 and 2 are using UE-specific precoding, theprecoding vector used for AP 7 is selected to provide precoding gaintowards the UE receiving the ePDCCH in eREGs 1 and 2. For the ePDCCHusing eREGs 3 and 4, the precoding vector of AP 7 is thus alreadydetermined. But, antenna port diversity can be obtained if the eNBselects the precoding vector for AP 8 to be different than the one usedfor AP 7. Preferably this is a precoding vector that is orthogonal tothe one used for AP 7.

With this arrangement, the PRB pair can be divided into eight eREGs thatcan be freely used for either diversity or UE-specific precoding forgroups of two eREGs. It should be noted also that each pair of two eREGsconsists of 36 REs, in this example, which equals the size of aconventional CCE. Hence, for each CCE mapped to a PRB pair, eitherUE-specific precoding or diversity transmission can be used, and theantenna port association can still be implicit based on the actual eREGsthat are used. This removes the need for antenna port signaling to a UE,which is a benefit of the solution in this embodiment. This also meansthat whether UE-specific precoding or diversity transmission is usedonly impacts a single CCE, and is thus self-contained within the CCE. Itshould be clear that these advantages are not limited to the particulardefinitions of eREG and CCE used in the illustrative examples describedabove, but may apply to other arrangements as well.

Given the approach described above, search space design becomes greatlysimplified, since both localized and distributed transmissions can bedefined in a given set of N_CCE-ePDCCH CCEs, by using differentaggregations of the available eREGs. Note that localized transmissionscommonly use UE-specific precoding while distributed transmissions mayuse either UE-specific or diversity transmission, but frequently usediversity transmission. Accordingly, the flexible approach describedabove for allowing both UE-specific precoding and diversitytransmissions in the same PRB pair translates directly into acorresponding flexibility with respect to supporting both localizedtransmissions and distributed transmissions using eREGs in a given PRBpair.

As shown above, a UE can be configured to use UE-specific precoding ordiversity transmission with respect to an ePDCCH, and transmissions fora given UE can be multiplexed, in the same PRB pair, with transmissionsfor other UEs, where each of the transmissions can independently beconfigured for a UE-specific or diversity transmission. It is then afurther problem how to define the search space to accommodate bothlocalized and distributed transmissions and transmissions withUE-specific and diversity. This is addressed in the second embodimentdescribed below, which second embodiment can advantageously be combinedwith the solutions described above, or used independently thereof.

With respect to this second embodiment, consider that localizedtransmission implies that eREGs and CCEs within the same PRB pair orwithin at most two PRB pairs are aggregated to form an ePDCCH, whiledistributed transmission implies that the ePDCCH candidate consists ofeREGs and CCEs aggregated from multiple PRB pairs.

Allowing eREGs/CCEs from within a given PRB pair to be flexibly used foreither localized or distributed transmission can be managed by, forexample, defining a matrix where each element is a CCE (or eREG or eCCEor any other name that implies a set of REs in the PRB pair) in a givenPRB pair. While the following discussion assumes that a CCE consists oftwo eREGs, it should be appreciate that this is a non-limiting exampleof just one of the possible groupings of REs.

FIG. 19 illustrates an example of such a matrix, for an example wherethe search space comprises K_ePDCCH=8 PRB pairs, with 4 CCE per PRB pairdefined. Hence, there are N_CCE-ePDCCH=32 CCEs in this example. Theindices of the used PRB pairs are 0, 1, 32, 33, 64, 65, 98 and 99 asshown in FIG. 19. An aggregation of CCEs across two or more of the PRBpairs, in a horizontal direction, provides a distributed transmission.Localized transmissions may be performed using aggregations of severalCCEs within a single PRB pair, or using several CCEs within two adjacentPRB pairs, where the CCEs are aggregated in a generally verticaldirection. Thus, FIG. 19 illustrates a two-dimensional search space thatencompasses both localized (L) and distributed (D) aggregations of CCEs.Two ePDCCH candidates with distributed transmission are shown in FIG.19, with AL=8 and AL=2, respectively.

The CCEs available in the search space for the UE can thus beillustrated by a 4×8 matrix, in this example, where distributedtransmission implies aggregation of CCEs in the horizontal direction andlocalized transmission implies aggregation of CCEs in the verticaldirection. The UE is configured so that a given ePDCCH transmission useseither UE-specific or diversity transmission, and can thus associate theeREGs to the proper antenna ports according to the first embodimentdescribed above. This configuration can be achieved by RRC signaling, insome embodiments. In some cases, this configuration can be dependent onwhich search space the ePDCCH belongs to. For instance, when an ePDCCHis transmitted in the common search space, then diversity transmissionmay be always assumed, in some embodiments. Note that this approach alsoallows a distributed transmission to utilize UE-specific precoding,which is useful if the preferred precoding information for a given UE isavailable at the transmitted base station but frequency diversitytransmission is still preferable. One example is when wideband precodingand wideband channel quality information (CQI) are available.

One potential problem with the solution described above in the secondembodiment is that an AL=8 distributed transmission blocks all localizedtransmissions with AL>2. This problem is further addressed by a thirdembodiment described below. This third embodiment can be combined withany or all of the previous ones, or used independently thereof.

To achieve maximal frequency diversity, an ePDCCH should preferably bemapped to all K_ePDCCH configured PRB pairs for the UE. It has beenagreed in 3GPP that whenever an ePDCCH is using a PRB pair, then PDSCHcannot be transmitted in that PRB pair. It is then a problem that whenonly a few of the eREGs per PRB pair are used, which may frequently bethe case when the ePDCCH load is low, most of the eREGs will then beempty. This leads to substantial and unnecessary control channeloverhead.

Furthermore, it is beneficial that a UE can receive an ePDCCH from onetransmission point within a cell and another ePDCCH from anothertransmission point within the same cell. For instance, a common searchspace may be transmitted from a transmission point with high power(macro) while the UE-specific search space may be transmitted from a lowpower node (sometimes called a “pico node”) in the same cell. FIG. 20illustrates an example where a UE may receive ePDCCH transmissions fromeither a macro or a pico node, or simultaneously from both nodes. Toaccomplish this, two different PRB groups are used for the two nodes.

Another possible scenario is that downlink assignments are transmittedfrom one transmission point while uplink grants are transmitted fromanother transmission point. In this scenario, as in the previous one, itis a problem how to co-ordinate, between the transmission points, theePDCCH transmissions to the same UE in the same subframe. This isespecially a problem if fast backhaul is not available, as quickcommunication between the transmitting nodes is then not possible. It isa further problem in these example scenarios that the received powerlevels between the ePDCCHs received from different transmission pointsmay differ significantly, for instance by 16 dB or more. This could leadto difficulties in designing hardware to receive those transmissionswhen they use adjacent resource elements, due to signal leakage betweenresource elements.

One solution to the above mentioned problems of control overhead andePDCCH reception from different transmission points is to ensure thatany given ePDCCH is mapped to only a subset of the K_ePDCCH configuredPRB pairs. For example, these K_ePDCCH PRB pairs could be divided intoPRB pair groups of at most four PRB pairs each, where any given ePDCCHis confined to one of these groups.

At low control signaling load, it can therefore be ensured that not allK_ePDCCH configured PRB pairs are used for ePDCCH. In this case, one ormore of them can instead be used for PDSCH transmissions, thus reducingthe control channel overhead. Furthermore, different transmission pointscan use different ones of these of these four-PRB-pair groups. (Notethat the number four is a non-limiting example). Hence, a fast backhaulis not needed to coordinate the ePDCCH transmissions betweentransmission points, as the PRB pairs can be assigned to the differenttransmission points according to these groups. This can be a semi-staticconfiguration that does not need fast backhaul between transmissionpoints.

Having transmissions separated into PRB-pair groups allocated totransmission points also reduces the potential problems ofreception-power imbalance. Note that since DMRS is used for ePDCCHdemodulation, the UE is not aware that ePDCCH in a first group of PRBpairs are transmitted from one transmission point while a second groupis transmitted from a second transmission point. Note also that it isassumed the search space for the UE encompasses multiple groups.

In one variation of this approach, the search space for each subframe isrestricted to one group of PRB pairs or a subset of the groupsconfigured for the UE. Hence, the UE searches ePDCCH candidates indifferent groups, in different subframes. For instance, downlinkassignments can be received in some subframes from one transmissionpoint, using one group of PRB pairs, while uplink grants are received inanother subframe, from another transmission point, using a differentgroup of PRB pairs. This totally removes the need for reception ofePDCCH of large transmission power differences in one and the samesubframe, which is a benefit that could simplify hardware implementationand cost.

An example of the approach described above, in which the PRB pairs aredivided into groups, is illustrated in FIG. 21, which shows a series ofmatrices containing the search space for a UE. In this example, each 4×4square of CCEs corresponds to one group of PRB pairs. Here, PRB pairnumber {0, 32, 64, 98} belongs to PRB pair group 1 and {1, 33, 65, 99}belong to group 2. Each PRB pair has four CCEs in this example. An AL=8transmission is shown in group 1 (using CCEs shown with shading), mappedpredominantly in the horizontal direction. This AL=8 transmission usestwo CCEs per PRB pair, and spans four PRB pairs. Hence, the AL=8transmission is distributed, but is confined within one PRB pair group.Furthermore, Group 2 in this example is used for an AL=8 localizedtransmission (mapped predominantly vertically) and an AL=2 distributedtransmission (mapped horizontally).

Note that if only the ePDCCH shown in Group 1 is transmitted in thesubframe, then all the PRB pairs of group 2 are unused and can thus beused for PDSCH transmissions. This is not possible if an AL=8 ePDCCHtransmission spans all 8 PRB pairs (i.e., using one CCE/PRB pair) as isthe case in the example shown in FIG. 19. Furthermore, when combinedwith the flexible eREG/CCE-to-antenna associations described earlier,two-fold antenna diversity can be achieved within each PRB pair(actually within each CCE). Accordingly, the AL=8 transmission shown inFIG. 21 achieves a total of eighth-order diversity.

The antenna diversity can, in the AL=8 case, be further increased if thetwo CCEs in each PRB pair are taken from the upper and lower subsets of4 eREGs, as shown in FIGS. 16-18, since these subsets use differentantenna ports. For instance the first CCE in one of the PRB pairs shownin FIG. 21 may use eREGs 1 and 2, which are mapped to antenna ports 7and 8, respectively, while the second CCE in that same PRB pair useseREGs 5 and 6, mapped to antenna ports 9 and 10. In this manner,fourth-order diversity can be obtained within a PRB pair, resulting in16th-order diversity for an AL=8 ePDCCH in total. FIG. 22 illustrates anexample of this approach, where CCEs (eREG groups) are interlaced withineach PRB pair to ensure that an AL=8 ePDCCH (shown with crosshatching)is mapped to eREGs that use different antenna ports, so that fourthorder antenna diversity is obtained within each PRB pair.

In the arrangements shown in FIGS. 21 and 22, the PRB pairs within agiven group are arranged in increasing order according to their indices,and are thus arranged according to increasing frequency. (Of course, itwould also be possible to arrange them in the reverse order.) The PRBsare also assigned to the groups in an interlaced fashion, so that, forexample, PRB pair 0 is in group 1, while PRB pair 1 is in group 2. Thishas the advantage that two adjacent CCEs in the horizontal directionwithin a given group have a large PRB spacing, and better frequencydiversity is thus obtained for the diversity mapping. An example is theAL=2 mapping in FIG. 21, where PRB pairs {1, 33} are used. In avariation of this approach, the order of the PRB pairs within each groupis further re-arranged to achieve even larger frequency diversity, whichcan be useful in flat channels and/or when the total bandwidth is small.An example of this is seen in FIG. 23, where two AL=2 ePDCCHtransmissions use PRB pairs {0, 64} and {32, 98}, respectively.

In a further variant, the number of PRB pairs per group may bedifferent. Furthermore some groups may contain only localizedtransmission, while others may contain both localized and distributedtransmission. An example is given in FIG. 24, where the second grouponly has three PRB pairs and can be used for localized transmissionand/or distributed transmission. Distributed transmission, of course, islimited to AL=2, in this example. Localized transmission of up to AL=8can be achieved in this example, and in any of the previous examples, bymapping the ePDCCH to two PRB pairs, since one PRB pair can encompassonly 4 CCE.

Following is a detailed description of a fourth embodiment, which can becombined with any or all of the previous approaches or usedindependently thereof. When the PRB pair used for ePDCCH transmissionscollides with transmissions of CRS, CSI-RS, PSS, SSS, PBCH or PRS, thenpuncturing of the ePDCCH may occur, such that the REs used to carrythese colliding signals are not available for ePDCCH mapping.Furthermore, the beginning OFDM symbol to be used for carrying ePDCCH ina given subframe could be configured to be different from the first OFDMsymbol in the subframe, since the first n=1, 2, 3, or 4 OFDM symbolscontain the legacy control channels (PDCCH, PHICH and PCFICH). In bothcases, REs are effectively removed from the eREGs, and the actual numberof available REs that can be used for mapping and transmission of themodulated ePDCCH symbols in an ePDCCH is smaller than in the nominal,non-punctured case. (Note that the nominal, non-punctured case onlyexists on new carrier types without CRS or legacy control regions). Tomaintain a consistent level of performance, this puncturing must becompensated for with link adaptation, by adjusting the aggregation levelfor the transmitted ePDCCH. Since the level of puncturing may depend onthe subframe number, a given ePDCCH may require different aggregationlevels in different subframes, even where the DCI payload and channelconditions are the same.

Due to this puncturing, the set of available aggregation levels {1, 2,4, 8} permitted in Release 8 of the LTE specifications may not besufficient to provide adequate link adaption. For instance, if 50% ofthe REs used for ePDCCH are punctured in a given subframe, then usingAL=8 is effectively an AL=4 transmission. This effective reduction inaggregation level reduces the coverage of the ePDCCH, which is aproblem.

According to the fourth embodiment detailed here, the set of availableaggregation levels depends on the level of puncturing in the subframe,and is known to the eNB and UE. Note that the level of puncturing mayvary from one subframe to another, and thus the available AL set alsodepends on the subframe in this case. In one example, when there is 50%puncturing then the set of AL is {2, 4, 8, 16}. An aggregation level ofAL=16 can be achieved by either increasing the number of CCEs used perPRB pair, or by using more PRB pairs. FIG. 25 illustrates two examples,each of which is based on the same two groups of PRB pairs that wereshown in FIG. 21. The left-hand side of FIG. 25 shows an example whereAL=16 is achieved by using more PRB pairs, including PRB pairs from bothgroups. The right-hand side of FIG. 25 shows an example in which AL=16is instead achieved by increasing the number of CCE used per PRB pair.

In the event that the level of puncturing is less than 50% for a givensubframe, for example, then a different set of aggregation levels isavailable according to this approach. For instance, the set of availableaggregation levels in this case could be {1, 2, 6, 12}. The number ofPRB pairs within a group might also be increased, as seen in the exampleshown in FIG. 26. In this example, with less than 50% puncturing, whereAL=12 is the maximal required AL, then six PRB pairs can be configuredfor group 1, with four CCEs per PRB pair. Aggregation levels{1,2,3,6,12} can be readily accommodated in this arrangement. In thisexample, group 2 might be used for localized transmission only, and/orfor distributed transmissions of AL={1, 2}.

According to this technique, then, the aggregation levels available forforming ePDCCH's may vary from subframe to subframe, based on thepuncturing level within the subframe. It will be appreciated that thistechnique is not limited in its application to the particulararrangement and definitions of eREGs and CCEs used above, but may beapplied to other arrangements as well. Furthermore, the sets ofaggregation levels and the threshold level of 50% used above are merelyexamples; other sets may be used, particularly if the number of REsand/or eREGs per CCE varies from what is assumed in the precedingexamples.

Any of the techniques discussed above may be combined with a fifthembodiment, which is detailed here. Since the common search space (CSS)is read by many or all UEs in the cell, it is a problem how to configurethe PRB pairs for ePDCCH transmissions so that the UEs monitor the sameCCEs for the common search space. Furthermore, monitoring of CSS in theePDCCH may be UE-specifically configured, as some but not all UEs maymonitor CSS in the PDCCH. Hence, it is beneficial if the CSS usesseparate PRB pair resources than the UE-specific search space. Note thatthis also allows transmission of CSS from a different transmission point(macro) using ePDCCH than the UE-specific search space, which could betransmitted from a pico or low power node. (See the third embodimentdiscussed above.)

These problems can be addressed by, for example, assigning one group ofPRB pairs to be a primary group, which is defined as primary since itcontains the CSS. This primary group is configured to include the samePRB pairs for all UEs that receive the same CSS in the cell. Additionalsecondary groups can then by configured more flexibly and so that theyare specific to one UE or a subset of UEs in the cell. This is useful toprovide interference coordination among cells and transmission pointsfor the UE-specific search space (USS), while maintaining macro coveragefor the CSS.

Furthermore, an identification of the PRB pairs used for the primarygroup (e.g., the PRB indices and the number of PRB pairs, which maydepend on the amount of puncturing according to the embodiment discussedabove), can be signaled using Radio Resource Control (RRC) signaling, orcan be signaled using previously unassigned bits in the PBCH message.The use of RRC signaling can be used at handover between cells, or whenconfiguring an additional non-backward compatible carrier type. In thiscase, a configuration message received on the primary cell indicates thePRB pairs used for the primary group of an additional non-backwardcompatible new carrier type.

Note that is also possible to use the PBCH to indicate the location andsize of the primary group of PRB pairs containing the CSS. This isparticularly useful when a stand-alone carrier is used, where initialaccess to a carrier that does not have PDCCH transmissions is performed.

Finally, note that all of the embodiments described above for ePDCCHtransmissions in above can also be applied to HARQ-ACK/NACKtransmissions in response to uplink PUSCH transmissions. This can bedenoted an enhanced PHICH (ePHICH) transmission.

The above described embodiments can provide one or more of the followingadvantages. First, these techniques can be used to reduce controlchannel overhead when ePDCCH is used. Second, several of the embodimentsmay be employed to facilitate the reception of ePDCCH from differenttransmission points in a transparent manner, and/or to facilitaterelatively slow backhaul coordination between transmission points usedfor different ePDCCH transmissions to the same UE. Further, several ofthe techniques enable a flexible adaptation of different degrees ofePDCCH puncturing in different subframes, by the use of aggregationlevel set adaptation Still others enable a flexible use of a CCE foreither UE-specific or diversity transmission.

One consequence of this last advantage is that the resources in a singlePRB pair can be used by the base station for both frequency-localizedtransmissions as well as distributed transmissions. FIG. 27 is a processflow diagram illustrating an example method according to this approach,as might be implemented in a base station like the one pictured in FIG.28. As shown at block 2710 of FIG. 27, the illustrated method fortransmitting downlink control information in a radio communicationsystem begins with transmitting a first control message to a first UE,using a first PRB pair. This first control message is divided among twoor more first non-overlapping subsets of resource elements, at least oneof which is in the first PRB pair, where the two or more firstnon-overlapping subsets of resource elements are aggregated in afrequency-localized manner within at least the first PRB pair. Symbolsin these two or more first non-overlapping subsets of resource elementsare transmitted using a single antenna port. As shown at block 2720, thebase station also transmits a second control message to a second UE,also using the first PRB pair. The second control message is alsodivided among two or more second non-overlapping subsets of resourceelements, at least one of which is in the first PRB pair. In this case,however, the two or more second non-overlapping subsets of resourceelements are aggregated in a frequency-distributed manner across thefirst PRB pair and one or more additional PRB pairs, and symbols in atleast two of the two or more second non-overlapping subsets of resourceelements are transmitted using differing antenna ports, these differingantenna ports including the single antenna port used to transmit thesymbols in the two or more first non-overlapping subsets of resourceelements. It will be appreciated that each of the non-overlappingsubsets of resources may define an enhanced REG (eREG) in a Long-TermEvolution (LTE) network, in some embodiments, although other groupingsor names for groupings may be used.

FIG. 28 is a process flow diagram illustrating a corresponding methodfor implementation in a UE. As shown at block 2810, the UE monitors afirst group of PRB pairs. This group of PRB pairs makes up a searchspace; in some embodiments this first group may be a user-specificsearch space. The monitoring of the first group includes forming each oftwo or more first candidate downlink control messages (e.g., candidateePDCCH's)_by aggregating two or more non-overlapping subsets ofresources from the first group of PRB pairs in a frequency-localizedmanner and demodulating each first candidate downlink control messageusing reference symbols corresponding to a single antenna port.Accordingly, a downlink control channel message sent to the UE in thisfirst search space may be precoded using a UE-specific precoding vector.

As shown at block 2820, the UE also monitors a second group of PRBpairs, in the same subframe. The monitoring of the second group includesforming each of two or more second candidate downlink control messagesby aggregating two or more non-overlapping subsets of resources from thefirst group of PRB pairs in a distributed manner and demodulating eachsecond candidate downlink control message using reference symbolscorresponding to at least two antenna ports. Accordingly, a downlinkcontrol channel message sent to the UE in this first search space may betransmitted using diversity transmission.

In some embodiments, the first and second groups of PRB pairs have atleast a first PRB pair in common. The monitoring of the first and secondgroups of PRB pairs may include using at least one subset of resourcesfrom the first PRB pair in one of the first candidate downlink controlmessages and using at least one subset of resources from the first PRBpair in one of the second candidate downlink control messages. In otherwords, a localized message candidate and a distributed message candidatemay use resources from the same pair, in the overlapping portion of thesearch spaces.

Of course, in some embodiments the non-overlapping subsets of resourceseach define an eREG in a LTE network, although other names and/orarrangements of these subsets may be used. Likewise, in someembodiments, the message candidates may be candidate ePDCCH messages,although again other names may be used.

As noted above, e.g., in connection with describing the process flows ofFIGS. 27 and 28, the various methods described herein can be performedby base stations and mobile terminals (e.g., LTE UEs). FIG. 29 is ablock diagram that illustrates relevant components of a generalized basestation adapted to carry out one or more of these methods, such as amethod according to the process flow of FIG. 27. As seen in FIG. 29,base station 2900 can include one or more processors 2902, which controloperation of other elements of the base station 2900, e.g., by runningsoftware or applications stored in one or more memory devicesrepresented by memory unit 2904. The base station 2900 will alsotypically include one or more receiver chains (RX) 2906 and transmitchains (TX) 2908 (collectively, one or more transceivers) adapted toreceive and transmit radio signals, respectively, over an air interface2912, via one or more antennas 2910. These radio signals include, forexample, the downlink control signals as described above.

In addition to circuitry for transmitting and receiving over the airinterface, for communicating with UEs, base station 2900 typically alsoinclude other network node interfaces 2914, e.g., an S1 interfaceadapted for communications with a Mobility Management Entity (MME) and aServing Gateway (SGW), and an X2 interface adapted for communicationswith other base stations. These interfaces can be implemented inhardware or a combination of hardware and software.

Many of the techniques described and illustrated above also impact theoperation of mobile terminals (UEs) that receive downlink controlsignaling from the base station. For instance, the manner in which theUE processes such downlink control signaling, searches for such downlinkcontrol signaling, and/or receives information associated with suchprocessing or searching is affected by the techniques. FIG. 30 is ablock diagram illustrating components of an example UE, adapted to carryout one or more of the techniques detailed above, such as the techniqueillustrated in the process flow of FIG. 28. As seen in the figure, UE3000 includes a processor 3002 connected to a memory unit 3004, whichmay store applications, programs or software for execution by theprocessor 3002. The processor 3002 may be configured to operate, inconjunction with one or more receive chains (RX) 3006, to receivedownlink control signals and/or associated information as describedabove. The UE 3000 also includes one or more transmit chains 3008 (TX),which collectively of the RX unit(s) may be referred to as“transceiver(s)”. The UE 3000 also includes one or more antennas 3010which can be used by the RX and/or TX units to receive/transmit radiosignals.

The method steps performed by the base station and/or UE, such as themethod steps illustrated in FIGS. 27 and 28, respectively, are performedby functional elements of the processing circuitry in the respectiveapparatuses. In some embodiments these functions are carried out byappropriately programmed microprocessors or microcontrollers, alone orin conjunction with other digital hardware, which may include digitalsignal processors (DSPs), special-purpose digital logic, and the like.Either or both of the microprocessors and digital hardware may beconfigured to execute program code stored in memory. Again, because thevarious details and engineering tradeoffs associated with the design ofbaseband (and other) processing circuitry for wireless base stations arewell known and are unnecessary to a full understanding of the invention,additional details are not shown here.

Program code stored in the memory circuit may comprise one or severaltypes of memory such as read-only memory (ROM), random-access memory,cache memory, flash memory devices, optical storage devices, etc., andincludes program instructions for executing one or moretelecommunications and/or data communications protocols, as well asinstructions for carrying out one or more of the techniques describedherein, in several embodiments. Of course, it will be appreciated thatnot all of the steps of these techniques are necessarily performed in asingle microprocessor or even in a single module.

The present invention may, of course, be carried out in other ways andin other combinations than those specifically set forth herein withoutdeparting from essential characteristics of the invention. The presentembodiments are to be considered in all respects as illustrative and notrestrictive. With these and other variations and extensions in mind,those skilled in the art will appreciate that the foregoing descriptionand the accompanying drawings represent non-limiting examples of thesystems and apparatus taught herein for facilitating load balancing in adata packet network. As such, the present invention is not limited bythe foregoing description and accompanying drawings. Instead, thepresent invention is limited only by the following claims and theirlegal equivalents.

1-18. (canceled)
 19. A method for transmitting downlink controlinformation in a radio communication system, the method comprising:transmitting a first control message to a first user equipment (UE) in afirst subframe and using a first Physical Resource Block (PRB) pair,wherein said first control message is divided among two or more firstnon-overlapping subsets of resource elements, at least one of which isin the first PRB pair, wherein the two or more first non-overlappingsubsets of resource elements are aggregated in a frequency-localizedmanner within at least the first PRB pair, and wherein symbols in thetwo or more first non-overlapping subsets of resource elements aretransmitted using a single antenna port; and transmitting a secondcontrol message to a second UE, also in the first subframe and alsousing the first PRB pair, wherein said second control message is dividedamong two or more second non-overlapping subsets of resource elements,at least one of which is in the first PRB pair, wherein the two or moresecond non-overlapping subsets of resource elements are aggregated in afrequency-distributed manner across the first PRB pair and one or moreadditional PRB pairs, and wherein symbols in at least two of the two ormore second non-overlapping subsets of resource elements are transmittedusing differing antenna ports, said differing antenna ports includingthe single antenna port used to transmit the symbols in the two or morefirst non-overlapping subsets of resource elements.
 20. The method ofclaim 19, wherein each of the non-overlapping subsets of resourcesdefines an enhanced REG (eREG) in a Long-Term Evolution (LTE) network.21. The method of claim 19, further comprising selecting a firstprecoder for use in transmitting the first non-overlapping subsets ofresources to provide precoding gain towards the first user equipment andsubsequently selecting a second precoder, differing from the firstprecoder, for use in transmitting some of the second non-overlappingsubsets of resources.
 22. The method of claim 21, wherein said secondprecoder is selected to be orthogonal to the first precoder.
 23. A basestation apparatus arranged to transmit downlink control information in aradio communication system, the base station apparatus comprisingtransmitter circuits and receiver circuits adapted to receive andtransmit radio signals over an air interface, and one or more processingcircuits adapted to: transmit a first control message to a first userequipment (UE) in a first subframe and using a first Physical ResourceBlock (PRB) pair, wherein said first control message is divided amongtwo or more first non-overlapping subsets of resource elements, at leastone of which is in the first PRB pair, wherein the two or more firstnon-overlapping subsets of resource elements are aggregated in afrequency-localized manner within at least the first PRB pair, andwherein symbols in the two or more first non-overlapping subsets ofresource elements are transmitted using a single antenna port; andtransmit a second control message to a second UE, also in the firstsubframe and also using the first PRB pair, wherein said second controlmessage is divided among two or more second non-overlapping subsets ofresource elements, at least one of which is in the first PRB pair,wherein the two or more second non-overlapping subsets of resourceelements are aggregated in a frequency-distributed manner across thefirst PRB pair and one or more additional PRB pairs, and wherein symbolsin at least two of the two or more second non-overlapping subsets ofresource elements are transmitted using differing antenna ports, saiddiffering antenna ports including the single antenna port used totransmit the symbols in the two or more first non-overlapping subsets ofresource elements.
 24. The base station apparatus of claim 23, whereineach of the non-overlapping subsets of resources defines an enhanced REG(eREG) in a Long-Term Evolution (LTE) network.
 25. The base stationapparatus of claim 23, wherein the processing circuits are furtheradapted to select a first precoder for use in transmitting the firstnon-overlapping subsets of resources to provide precoding gain towardsthe first user equipment and to subsequently select a second precoder,differing from the first precoder, for use in transmitting some of thesecond over-lapping subsets of resources.
 26. The base station apparatusof claim 25, wherein the processing circuits are adapted to select thesecond precoder to be orthogonal to the first precoder.
 27. A method ina user equipment (UE) for receiving downlink control information in aradio communication system, the method comprising, for a first subframe:monitoring a first group of physical resource block (PRB) pairs, whereinsaid monitoring of the first group comprises forming each of two or morefirst candidate downlink control messages by aggregating two or morenon-overlapping subsets of resources from the first group of PRB pairsin a frequency-localized manner and demodulating each first candidatedownlink control message using reference symbols corresponding to asingle antenna port; and monitoring a second group of PRB pairs, whereinsaid monitoring of the second group comprises forming each of two ormore second candidate downlink control messages by aggregating two ormore non-overlapping subsets of resources from the second group of PRBpairs in a distributed manner and demodulating each second candidatedownlink control message using reference symbols corresponding to atleast two antenna ports.
 28. The method of claim 27, wherein the firstand second groups of PRB pairs have at least one common PRB pair, andwherein said monitoring of the first and second groups of PRB pairscomprises using at least one subset of resources from the common PRBpair in one of the first candidate downlink control messages and usingat least one subset of resources from the common PRB pair in one of thesecond candidate downlink control messages.
 29. The method of claim 28,wherein said monitoring of the first and second groups of PRB pairscomprises using first channel estimates, corresponding to the singleantenna port, for demodulating the one of the first candidate downlinkcontrol messages and using the first channel estimates as well as secondchannel estimates, corresponding to an additional antenna port, fordemodulating the one of the second candidate downlink control messages.30. The method of claim 27, wherein each of the non-overlapping subsetsof resources defines an enhanced REG (eREG) in a Long-Term Evolution(LTE) network.
 31. The method of claim 27, wherein the first group ofPRB pairs makes up a user-specific search space and the second group ofPRB pairs makes up a common search space.
 32. A user equipment apparatusarranged to receive downlink control information in a radiocommunication system, the user equipment apparatus comprisingtransmitter circuits and receiver circuits adapted to receive andtransmit radio signals over an air interface and further comprising oneor more processing circuits adapted, for a first subframe, to: monitor afirst group of physical resource block (PRB) pairs, such that saidmonitoring of the first group comprises forming each of two or morefirst candidate downlink control messages by aggregating two or morenon-overlapping subsets of resources from the first group of PRB pairsin a frequency-localized manner and demodulating each first candidatedownlink control message using reference symbols corresponding to asingle antenna port; and monitor a second group of PRB pairs, such thatsaid monitoring of the second group comprises forming each of two ormore second candidate downlink control messages by aggregating two ormore non-overlapping subsets of resources from the second group of PRBpairs in a distributed manner and demodulating each second candidatedownlink control message using reference symbols corresponding to atleast two antenna ports.
 33. The user equipment apparatus of claim 32,wherein the first and second groups of PRB pairs have at least a firstPRB pair in common, and wherein the processing circuits are adapted tomonitor of the first and second groups of PRB pairs in such a way thatat least one subset of resources from the first PRB pair is used in oneof the first candidate downlink control messages and at least one subsetof resources from the first PRB pair is used in one of the secondcandidate downlink control messages.
 34. The user equipment apparatus ofclaim 33, wherein the processing circuits are adapted to monitor thefirst and second groups of PRB pairs by using first channel estimates,corresponding to the single antenna port, for demodulating the one ofthe first candidate downlink control messages and using the firstchannel estimates as well as second channel estimates, corresponding toan additional antenna port, for demodulating the one of the secondcandidate downlink control messages.
 35. The user equipment apparatus ofclaim 32, wherein each of the non-overlapping subsets of resourcesdefines an enhanced REG (eREG) in a Long-Term Evolution (LTE) network.36. The user equipment apparatus of claim 32, wherein the first group ofPRB pairs makes up a user-specific search space and the second group ofPRB pairs makes up a common search space.